Magnesium has the advantage of being light weight and readily castable to complex forms. Thus, it would seemingly be a desirable material to use for aerospace applications. However, magnesium suffers from the disadvantage of being prone to corrosion. Corrosion of magnesium may produce corrosion pits, which can be deep and difficult to repair. Further, upon exposure to air above room temperature and at relatively low but elevated temperatures, magnesium may burn. For at least these reasons the use of magnesium in aerospace applications has been limited.
Various means have been proposed to address the above-noted disadvantages. For example, it has been proposed to cover magnesium components with coatings and/or paints as a way to improve corrosion protection for magnesium. However, coatings and paints can, in some instances, be relatively easily scratched. Moreover, a magnesium substrate with a coating may set up an electrical cell if the coating does get scratched, which can subsequently lead to rapid corrosion of the magnesium exposed by the scratch.
Magnesium is also susceptible to erosion and wear. Both these processes remove material that must be replaced during a repair. Otherwise the part may need to be scrapped. Thermal spray techniques employing other metals and materials have been used with other metals as a method to replace eroded material. However, thermal spraying is not well suited for use with magnesium. The temperatures involved to melt and bond the deposited metal in thermal spraying techniques can result in burning of the magnesium.
Aluminum has a low melting point and might thereof be considered a material that could be thermally sprayed to repair magnesium. However, there are problems in thermal spraying aluminum because it is prone to oxidation forming a tenacious oxide film. Even if it was possible to easily thermally spray aluminum, the phase diagram shows that it forms an intermetallic with magnesium at temperatures of approximately 450° C. and has significant solid solubility in magnesium. Thus, heretofore aluminum has not been considered a likely candidate for use in repairing magnesium.
Hence there is an ongoing need to provide protection for magnesium substrates against both corrosion and erosion. It is desired that these protective methods be stable and durable so as to avoid scratching. There is also a need to be able to repair corrosion pits and build up material lost to wear and erosion in magnesium parts. It is further desired to develop methods to apply these magnesium repair techniques on surfaces of cast magnesium parts used in aerospace applications such as those found in helicopter components. The present invention addresses one or more of these needs.